Conventional unipolar resonant tunneling hot-electron transistors (HETs) suffer from low dc current gain. Some reasons for this low dc current gain are 1) the base of a conventional HET must be thick in order to reduce the resistance of the base and 2) the base of a conventional HET must be heavily doped in order to prevent depletion of majority carriers when direct contact is made to the base. The present invention discloses a transistor that exhibits gain that is high enough for practical applications without requiring a thick and heavily doped base. Conventional HETs are also difficult to fabricate. The transistor of the present invention is much easier to fabricate.
The hot-electron transistor concept was first proposed by C. A. Mead in 1960. The HET initially used metal-oxide-metal (MOM) structures in a MOMOM configuration. The idea was based on the use of high-energy, and therefore, "hot," electrons, to perform high-speed amplification. But high-quality epitaxial metal and oxide films, required of this structure, proved difficult. Interest in HETs was revived when J. M. Shannon, in an article entitled "Hot electron camel transistor," IEE Journal of Solid State Electron Devices, vol. 3, pp. 142-144, 1971, proposed using semiconductor's in place of metal in the MOM structure. As a result of advanced crystal growth techniques, such as molecular beam epitaxy (MBE), the operation of HETs has been demonstrated in GaAs/AlGaAs and other heterojunction material systems. For example, Hayes et al., in an article entitled "Hot electron spectroscopy," Electron. Lett., vol. 20, pp. 851-852, 1984, used planar-doped GaAs barriers and observed three-terminal HET characteristics. Yokoyama et al., in an article entitled "Characteristics of double heterojunction GaAs/AlGaAs hot electron transistors," IEDM Tech. Dig., pp. 532-535, 1984, reported the first observation of current gain in tunneling-barrier HETS. Yokoyama et al., in an article entitled "A new functional resonant-tunneling hot electron transistor (RHET), Japan J. App. Phys., vol. 24, pp. L853-L854, 1985, reported the first use of a double-barrier resonant-tunneling structure to narrow the energy spread of tunneling hot-electrons. Heiblum et al., in an article entitled "Direct observation of ballistic transport in GaAs," Phys. Rev. Lett., vol. 55, pp. 2200-2203, 1985, disclosed a single-barrier AlGaAs tunneling-junction.
The operation of the HET is generally based on nonthermalized, hot electrons which are injected into the base layer of the HET. Only those electrons which are energetic enough will contribute to the collector current. But hot electrons thermalize efficiently, resulting in dc current gain that is often to small for practical applications. Because the energy relaxation process is so efficient, it is found experimentally and confirmed theoretically that a thin GaAs base with less doping is required to increase dc current gain for practical applications. However, contact to the thin base is usually made by etching to reach and reveal the base material. The result is that the Fermi-level pinning at the GaAs surface would completely deplete the base, if the base thickness is less than the depletion width. Because of these practical difficulties, there has been limited, but steady progress in the development of GaAs/AlGaAs-based HETs.
The present invention discloses a unipolar, three-terminal, hot-electron transistor. This invention is related to conventional HETs, but is different in the following important ways. First, the base is made much thinner (e.g., 100 angstroms) then the base of conventional HETs. As a result, the two lowest subbands are well separated. Second, present invention allows hot electrons to be injected directly into the second lowest subband. The injected electrons must either resonantly tunnel to the collector, or relax to the lowest subband in the base. In conventional HETs, the hot electrons only probe the electronic states in the base that do not show spatially quantized behavior. Third, the thin GaAs base can be electrically (i.e., non-physically) contacted rather than directly (i.e., physically) contacted as in conventional HETs. The present invention exhibits higher dc current gain per area and doping rate than does conventional HETs. This improvement in dc current gain is the result of using a thinner base and lower doping density than is used in conventional HETS.
Yang et al., in an article entitled "New field-effect resonant tunneling transistor: Observation of oscillatory transconductance," Appl. Phys. Lett. vol. 55, No. 26, pp. 2742-2744, Dec. 25, 1989, disclosed a resonant tunneling transistor that uses a double-barrier resonant tunneling structure. The present invention differs from the structure disclosed in Yang et al. in the following important ways. The present invention uses a thinner base (i.e., 100 angstroms instead of 200 angstroms) which results in higher dc current gain and, therefore, higher performance. The present invention is a three terminal device rather than a four terminal device as disclosed by Yang et al. The present invention contacts the quantum well (e.g., the base) electrically rather than by an alloying contact as disclosed by Yang et al.
U.S. Pat. No. 5,179,037, entitled INTEGRATION OF LATERAL AND VERTICAL QUANTUM WELL TRANSISTORS IN THE SAME EPITAXIAL STACK, discloses a method of fabricating different types of resonant-tunneling transistors on the same substrate and does not disclose the transistor of the present invention.
U.S. Pat. No. 4,959,696, entitled THREE TERMINAL TUNNELING DEVICE AND METHOD, discloses devices that 1) utilize an alloying contact or a tunneling contact, 2) require three different bandgaps, 3) use a single-barrier structure, and 4) utilize a thin collector. Instead, the present invention uses a tunneling contact, does not require three different bandgaps, uses a double-barrier resonant-tunneling structure, and uses a thick collector with a barrier height that is higher than the barrier height of the devices disclosed in U.S. Pat. No. 4,959,696.
U.S. Pat. No. 4,902,912, entitled APPARATUS INCLUDING RESONANT-TUNNELING DEVICE HAVING MULTIPLE-PEAK CURRENT-VOLTAGE CHARACTERISTICS, discloses a method of connecting a series of passive components in a resistor network. The resulting device does not exhibit transistor action as does the present invention.
U.S. Pat. No. 4,878,095, entitled SEMICONDUCTOR DEVICE IN PARTICULAR A HOT ELECTRON TRANSISTOR, discloses a hot-electron transistor essentially consisting of an n+ collector, an asymmetrically doped separating layer, and an undoped control electrode that is contacted directly. The present invention is very different. Where U.S. Pat. No. 4,878,095 uses an asymmetrically doped separating layer, the present invention uses an undoped insulating layer. Where U.S. Pat. No. 4,878,095 uses an undoped control electrode, the present invention uses a doped control electrode. Where U.S. Pat. No. 4,878,095 contacts the control electrode directly, the present invention contacts the control electrode indirectly (i.e., electrically).
U.S. Pat. No. 4,721,983, entitled THREE TERMINAL TUNNELING DEVICE, discloses a transistor consisting essentially of an n+ gate, an insulating layer, an undoped quantum well, a single-barrier tunneling structure, and an n+ drain and source. The present invention differs from this device in that the present invention utilized a doped quantum well (either n+ or p+) and a double-barrier tunneling structure.